When making laminates, it is conventional to utilize a plurality of resin-impregnated core sheets. The sheets employed for this purpose are usually prepared from cellulosic fibers, e.g. creped or uncreped kraft paper and the like. In common practice, the fibrous core sheet material, in the form of a continuous sheet is first impregnated with resin, usually a thermosetting synthetic resin and, more particularly, in the case of cellulosic core sheets, a thermosetting phenolic resin such as a phenol-formaldehyde resin, a cresolformaldehyde resin or the like. The resin-impregnated core sheet material is then dried to a desired volatile content and finally cut to the appropriate size.
Often times, laminates are prepared solely from a plurality of these resin-impregnated core sheets stacked in superimposed relationship, and the number of plies in the stack will depend on the use for which the laminate is intended. In most cases, however, one or more decorative overlayments are placed on top or on both top and bottom of such a core assembly prior to lamination.
Thus, for example, in preparing a conventional decorative laminating assembly, a print sheet, which usually comprises a single, resin-impregnated sheet of an absorbent high alpha-cellulose or regenerated cellulose paper or similar fibrous material bearing an ornamental design or dyed or pigmented to impart a solid color thereto, is placed on top of the core member, which in this case generally contains 5 to 8 plies. A protective overlay sheet, which is usually similar to the print sheet except for being undecorated, is then usually placed over the print sheet.
The resins used to impregnate the print and overlay sheets are generally thermosetting synthetic resins such as aminotriazine-aldehyde, e.g. melamine formaldehyde which do not develop any significant amount or undesirable discoloration when subjected to laminating temperatures.
Similarly, metal clad laminates are obtained from assemblies wherein a thin metal sheet or foil, e.g. a copper, an aluminum, a steel, and alloys such as brass is placed on top of the core member.
Laminating assemblies of this type may be individually laminated by application of heat and pressure thereto; however, for obvious economic reasons, it is common practice to consolidate a plurality of these individual laminating assemblies into one large assembly or press pack and then to laminate this pack in one operation.
Where individual laminating assemblies containing, besides the core member, overlayments such as those described above are concerned, and also where it is desired to obtain laminates having one major surface either smooth or textured and free of defects from a plurality of core sheets as the sole laminae, the press pack will be built from these individual laminating assemblies placed back-to-back.
In building such a pack, an individual laminating assembly is placed with its overlayment surface adjacent to a polished press plate. The core members are then placed on the overlayment and another individual laminating assembly is then positioned back-to-back with the first assembly, with a separator sheet being placed between the core members of the individual assemblies. Another polished press plate is placed on the second individual assembly adjacent to its overlayment surface. Thus, at this point, the pair of laminating assemblies can be considered as being in mirror image relationship between the press plates, separated only by the separator sheet.
In its simplest embodiment, a back-to-back press pack would consist of this arrangement of one pair of individual laminating assemblies between their core members. In actual commercial practice, however, the entire procedure is usually repeated many times, until a pack having the desired height has been built. The press pack is then subjected to heat and pressure by inserting it into a laminating press, to consolidate the individual laminating assemblies into unitary structures. When the press pack is removed from the laminating press, the resulting pairs of laminates, pressed back-to-back are removed from between the press plates and then separated from one another at the locus of the separator sheet.
As previously indicated, this multiple laminating method affords definite economic advantages. However, as practiced commercially at the present time, it also has certain inherent disadvantages. Foremost among these is the fact that in order to be truly effective, the required separator sheet must, in many cases, be prepared from relatively costly materials. A partial solution to this problem is described in U.S. Pat. No. 3,050,434 to Emily et al. which discloses a separator sheet comprising a web of paper coated with a film of a salt of alginic acid. It has been found that while such sheets may sometimes result in a satisfactory laminate release, more times than not the release will be unsatisfactory. The deficiency of such sheets results from the fact that the paper completely absorbs the alginic salt so that very little, if any, remains on their surface. Application of large amounts of alginic salt does not appear to improve the frequency of inferior releases.
A later advance in the art was made as described in U.S. Pat. No. 3,215,579 to Hagen. This patent discloses that a paper web, and particularly a saturating kraft paper web, can be inexpensively made into a separator sheet by first sizing the web with an aqueous solution of a water-soluble alkaline earth or earth metal salt and then coating the sized web on at least one side, i.e. with a film of a salt of alginic acid.
Although superior to the alginic salt alone, the sized release sheet also absorbs a great deal of sizing agent and alginic salt so that it, too, frequently results in an inferior release when used to separate decorative laminates undergoing consolidation. Large amounts of alginic salt, even applied in sequential layers does not improve these deficiencies. Only by incorporating a phenolic resin was Hagen able to produce a satisfactory release sheet. The use of such a resin, before sizing, is very costly.